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Friday, 13 November 2015

Two Tanystropheus longobardicus tussle in Triassic Europe. There's a distinct lack of water supporting their necks in this scene, and some might suggest this makes such behaviour impossible for these animals. But does it? Read on...

One of the most famous non-dinosaurian denizens of the Mesozoic is Tanystropheus, a spectacularly long-necked reptile which lived across Europe and Asia in Middle-Late Triassic times. We've known about this 5-6 m long animal since fragmentary fossils were pulled from Italian Triassic rocks in 1855, and now regard it as a particularly large and anatomically extreme member of the Protorosauria. This is a Permian-Triassic group of archosauromorphs (all reptiles more closely related to crocodylians and birds that lizards) that spawned numerous aberrant taxa, such as drepanosaurs, Sharovipteryx and Dinocephalosaurus. Within Protorosauria, Tanystropheus can be considered a tanystropheid, closely related to similar, but shorter-necked and smaller-bodied species such as Tanytrachelos and Langobardisaurus.Tanystropheus longobardicus is by far the best known Tanystropheus representative, and the one we always think of when discussing this animal, but something like five Tanystropheus species have been named over the years. It is currently uncertain how many of these should be considered valid and, of those, which ones truly represent Tanystropheus and not some other type of protorosaur. There are hints that longobardicus might be the sole representative of this genus, but work on this is ongoing.

We know a lot about Tanystropheus because it's fossils are not uncommon, and many of them are complete or every nearly so. Its remains occur in Alpine Europe, the Middle East and China and we can conclude that, weird as it seems, the Tanystropheus bauplan and life strategy was a successful one. But exactly what that strategy was remains a bone of contention for palaeontologists. Summarised simply, opinion is divided over whether Tanystropheus was confined to aquatic habits, at least above a certain age and body size, or else capable of living terrestrially as a shore-patrolling, 'animated fishing rod'. Unsurprisingly, the principle source of this contention is its neck anatomy: clearly long and relatively stiff in life, was it so heavy that would over-balance the animal if not supported in water? Or was the neck anatomy not as heavy as commonly supposed and really no great hindrance to life on land? Other aspects of Tanystropheus form have also influenced this debate, including limb structure and tail anatomy, but it seems fair to say these discussions persist because experts disagree about the significance of that crazy neck. Renesto (2005) and Nosotti (2007) provide recent overviews and contributions to this long-running controversy.

I've been wanting to cover Tanystropheus lifestyle here for some time now, and I've ended up with sufficient material to spread discussion over two posts. In the next article I'll be discussing nuances of arguments for aquatic and terrestrial habits, but, first, I want to satisfy some personal curiosity over how Tanystropheus was constructed. Specifically, I'm interested in the mass distribution of this animal: is it really _that_ front heavy? There are plenty of terrestrial animals with very long necks - sauropods, giraffes, some pterosaurs - and we don't worry about them toppling over. Moreover, although neck mass is frequently mentioned as critical to understanding Tanystropheus lifestyle, to my knowledge, there isn't any information available on its body volumes or mass (if I'm wrong, please tell me below). I thought I'd see what I could find out about this myself using the GDI (Graphic Double Integration) method of volumetric mass estimation, a quick and easy way to get a sense of mass and body fractions of fossil animals. It basically involves chopping up drawings of animals to determine volumes of body segments, then multiplying these by a suitable density - check out this excellent SV:POW! summary for a full lowdown.

Tanystropheuslongobardicus as reconstructed by Rupert Wild in 1973. Image borrowed from Palaeos.

GDI methods require a clear layout of animal form to measure and divide into segments. There is no shortage of life restorations of Tanystropheus out there, and plenty of photographs of near complete specimens, but objective, modern portrayals of its anatomy are hard to come by. Rupert Wild's skeletal reconstruction from the 1970s (above) seems to remain a common frame of reference, and a David Peters reconstruction is sometimes used as an alternative. Neither really seemed suited for my purposes here, the crouched poses obscuring anatomical details, some specifics of vertebral count being inaccurate to modern interpretations, and the latter being produced with techniques of questionable reliability. I decided to try my hand at producing a new skeletal reconstruction based on the large, near complete Tanystropheus skeleton described in detail by Rieppel et al. (2010): GMPKU-P-1527:

I wanted a large animal because the Tanystropheus neck seems to increase in length disproportionately to body size (Tschanz 1988). I want to give this animal the best chance of falling over, so it makes sense to use the largest neck possible. GMPKU-P-1527 is articulated and includes most of the neck, missing only the relatively short anterior 3.5 vertebrae (of 13), the skull, and the end of the tail. I reconstructed these missing parts using smaller Tanystropheus specimens (from Nosotti 2007) and Wild's widely-used 'adult' skull reconstruction. These came together to form a skeleton measuring 3.5 m as reconstructed, and likely over 4 m if the vertebral column were completely straightened. This is not as large as we think this animal could get, but is c. 70% of maximum size, and the minimal amount of proportional inference and cross scaling means we should be looking at a fairly authentic image of Tanystropheus form. The results are below.

Tanystropheus cf. longobardicus skeletal reconstruction, almost entirely based on GMPKU-P-1527. See text and illustration below for details on which bits are borrowed from other specimens.

The length of the limbs here is quite striking. Note that they aren't depicted in a true sprawling pose, because foreshortening would impact measurements for the mass calculation, but I depicted a crouched pose which hopefully conveys something of a low, sprawling gait. I also followed Nosotti's (2007) suggestion of digitigrady, which boosts the standing height a bit. Despite the low pose, I immediately get a different vibe from this image to that of Wild's classic, sitting reconstruction. Simply putting the animal on its feet gives the impression of the limbs and body being more proportionate to the neck. The arc of the neck follows that preserved in GMPKU-P-1527 quite closely, a pose also occurring in several other articulated Tanystropheus specimens. As depicted, I don't think it conflicts with recent interpretations of Tanystropheus neck arthrology (e.g. Renesto 2005). The body outline should be non-controversial, pretty much following the outline of the skeleton and hitting major muscle landmarks.

Time to chop this guy and up see what it's made of. Ideally, we'd want full orthographic views for a GDI mass estimate, but I've not had time to produce a multi-view skeletal. This means we're going to have to make predictions of body width. For the neck, body and tail, I decided to calculate width as 2/3 of body segment height, this being indicated by the proportions of Tanystropheus neck and tail verts, and the fact the dorsal ribs straighten out as they approach the lateral margins of the body. The 2/3 figure is a little arbitrary and arm-wavy, but seems more precise than assuming a circular cross section across the entire body. Other elements - the head and limbs - were modelled as having circular cross sections, however. You can see how I chopped the reconstruction up below: note that this uses an earlier, differently posed version of the skeletal shown above, and that the limbs are somewhat straighter. The bone sizes are no different, however, so influences on mass estimation should be negliable.

GDI mass estimation on Tanystropheus cf. longobardicus. Grey portions of the skeleton show which parts were modelled on other specimens. Numbers in parentheses give mass fractions for each body component, and the grey shapes indicate the cross-sectional shape used for that part of the body.

The entire animal shakes out to 26.7 L, and using a middle-of-the-road reptile density of 0.85 kg/L, the animal masses 22.9 kg. Of more interest to us is the mass percentages of each component, which are indicated in parentheses in the illustration above. You can see that the neck and head together are a hair away from 20% of the body mass, despite accounting for something like half the length of the animal. Virtually all of this 20% represents neck, of course, the head being less than 1% of the overall mass. As is usual for tetrapods, the trunk volume dominates all, being 50% of overall bulk despite only just exceeding 50 cm long in a 3.5 m long animal.

What do these figures actually mean? 20% doesn't seem like that much in the grand scheme of things, it being balanced by the other 80% of the body. These are certainly not values which make me think this animal perpetually toppled over unless it was in water. But can we be more precise here - how does this neck fraction stand up to other long-necked animals? For brachiosaurid sauropods, Mike Taylor (2009) suggested the neck accounted for 14% of the body mass, while Don Henderson (2010) suggested 8% for the same animals, noting that this was the largest neck mass fraction in his dataset of 10 volumetric sauropod mass predictions. Mitchell et al. (2013) did not report exact head and neck mass fractions for a large set of giraffes, but eyeballing their data suggests male giraffe necks and heads account for around 14% of body mass, with females slightly less than that. These are all significantly lower than the 20% I've estimated for Tanystropheus, implying that my gut feeling might be wrong: maybe it did have quite a heavy neck and, perhaps, was at greater risk of overbalancing.

However, it strikes me that giraffes and sauropods are not particularly good analogues for Tanystropheus, because their anatomy is built around a fundamentally different set of demands: processing of plant material. Herbivores need large guts to get the most from their nutrient-poor diet, equating to proportionally larger trunk volumes. Anyone who's played with volumetric mass estimations will know that even slight adjustments to trunk proportions can have a big impact on absolute mass and tissue fractions because they represent the biggest components of most animal bodies. We therefore can't ignore the requirement for herbivore torsos to be large when comparing them to non-herbivores like Tanystropheus. Our problem here is that finding a long-necked terrestrial carnivore to compare with Tanystropheus is challenging. Such body plans have been rare throughout geological time and are entirely unrepresented nowadays. We're not entirely licked, though: following the laws of monster movie science, any challenge involving a poorly understood, freakish creature is best solved with another poorly understood and freakish animal: in this case a long-necked azhdarchid pterosaur. Azhdarchid palaeoecology has a history of contention and controversy, but no-one believes that they were aquatic animals, or herbivorous, or at risk of toppling forward without environmental aid. This is despite azhdarchids bearing neck/trunk proportions similar to those of Tanystropheus, as well as much larger heads. We just assume they could carry their heads and necks one way or another, because all indications are that they were not adapted for an aquatic existence.

Taking a GDI approach to the Zhejiangopterus linhaiensis skeletal I produced earlier this year, I attempted to gather some data on azhdarchid body volumes and masses. As before, I estimated widths rather than producing full orthogonal views. The head and neck were assumed to be half as wide as tall, with the neck widths not permitted to exceed those of the skull. All other elements are treated as having circular cross sections. Azhdarchid torsos, forelimbs and necks are all highly pneumatised, so I gave these low tissue densities of 0.7 kg/L (about the lowest density recorded for modern birds), while the legs were given a more typical density of 0.85 kg/L. The breakdown and results:

Zhejiangopterus linhaiensis gets the GDI treatment. As above, numbers in parentheses indicate mass fractions, and the grey shapes indicate cross sectional area used in the calculations. Skeletal based on data in Cai and Wei (1994).

First things first, I was happy to see the animal come out at 7.9 kg - that's in line with most post-2000 interpretations of pterosaur mass, and seems about right for an animal with a 2.5-3 m wingspan. That makes me think the constituent volumes and masses are probably in a sensible ball park. In terms of body component masses, the torso - famously small in derived pterodactyloids like azhdarchids - provides less than 25%, the neck is just over 25%, and the head and paired forelimbs are 22% each. The legs account for just under 6%, and the tail might as well not exist. This puts - wow - almost 50% of the mass in front of the shoulders in this reconstruction. But even accepting that I've been generous with neck tissue in my reconstruction (following a reptilian, rather than avian pattern of neck musculature), and that some cross sectional shapes used here could be refined, it seems unlikely we could slim the head and neck tissues down to the mass fractions seen in long-necked herbivores. Even if my estimates are out by a factor of 2, the neck and head will still account for more mass than the same components in Tanystropheus. This finding makes the neck tissue fraction of the Tanystropheus model look a lot less aberrant, as well as verifying the suspicion that lifestyles, and not just anatomy, are important factors when comparing animal bauplans.

Let's bring all this together. While the sums outlined here are provisional, back-of-the-envelope-type stuff, I find them sufficient to at least make me sceptical of claims that Tanystropheus has a terrestrially-untenable mass distribution. At least one group of non-aquatic Mesozoic carnivores seem more front heavy, and a basic model of Tanystropheus mass distribution does not raise major alarm bells about relative neck and head weight. I could be convinced otherwise, and obviously there's a lot more than could - and should - be done to investigate this issue, but I currently don't see neck mass as a significant barrier to terrestrial habits. This exercise has also brought home the fact that we might not know much about the adaptive and structural significance of extremely long necks in carnivorous animals, and that we should be careful comparing them to other long-necked creatures. Perhaps our unfamiliarity with this extinct bauplan, along with our generally poor intuitive sense of animal mass and tissue fractions (see this discussion and comment field at SV:POW!), means we should be extra cautious about gut-feeling interpretations of such creatures. I guess the bottom line is that running numbers to test our intuitions is an essential part of understanding unfamiliar animal types, especially if we're suggesting those assumptions are significant for extinct animal behaviour and lifestyle.

There'll be more on Tanystropheus in the next post, where the plan is to review recent arguments for and against different lifestyles in this animal. In the mean time, I'm very curious to know what others make of the ideas presented here. Would you interpret these results differently? Would you have reconstructed Tanystropheus in a different way? The comment field is open...

Tanystropheus was brought to you by Patreon, a weekend of downtime and the letter 'S'

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Palaeontologist and palaeoartist, based at the University of Portsmouth and specialising in flying reptiles. Check out my work at MarkWitton.com, follow me on Twitter @MarkWitton, and buy my book here. Contact me about my work at wittonprints@gmail.com.